The operation of any scanning-head device in slewing the printhead across the medium to discharge ink droplets does present some obstacles to precise positioning of the printed marks, and also to best image quality. In order to describe these obstacles it will be helpful first to set forth some of the context in which these systems operate.
In many printing devices, position information is derived by automatic reading of graduations along a scale or so-called "encoder strip" (or sometimes "codestrip") that is extended across the medium. The graduations typically are in the form of opaque lines marked on a transparent plastic or glass strip, or in the form of solid opaque bars separated by apertures formed through a metal strip.
Such graduations typically are sensed electrooptically to generate an electrical waveform that may be characterized as a square wave, or more rigorously a trapezoidal wave. Electronic circuitry responds to each pulse in the wavetrain, signaling the pen-drive (or other marking-head-drive) mechanism at each pixel location--that is, each point where ink can be discharged to form a properly located picture element as part of the desired image.
These data are compared, or combined, with information about the desired image--triggering the pen or other marking head to produce a mark on the printing medium at each pixel location where a mark is desired. As will be understood, these operations are readily carried out for each of several different ink colors, for printing machines that are capable of printing in different colors.
In addition to this use of the encoder-derived signal as an absolute physical reference for firing the pens, the frequency of the wavetrain is ordinarily used to control the velocity of the pen carriage. Some systems also make other uses of the encoder signal--such as, for example, controlling carriage reversal, acceleration, mark quality, etc. in the end zones of the carriage travel, beyond the extent of the markable image region.
Now, standardized circuitry for responding to each pulse in the encoder-derived signal is most straightforwardly designed to recognize a common feature of each pulse. Thus some circuits may operate from a leading (rising) edge of a pulse, others from a trailing (falling) edge--but generally each circuit will respond only to one or the other, not both.
Such circuits have been developed to a highly refined stage. Accordingly it is cost-effective and otherwise desirable to employ one of these well-refined, already existing circuits relative to such compensation; however, in adapting such a preexisting design, several problems arise.
(a) Encoder dimensional tolerances--As noted earlier, the position of the pen carriage unit is monitored by a detector that reads a position encoder. In this regard, encoder-reading circuitry processes the encoder reader data to produce pulses each time the carriage unit moves a fixed distance, usually on the order of 1/75th or 1/1 50th of an inch. However, the drop placement on the print medium must be placed on spatial boundaries that are much smaller, such as down to 1/50,000th of an inch or even smaller. Unfortunately, the position encoder is not nearly this precise. Moreover, other problems are associated with the encoder dimensional tolerances. For example, the encoder-reading circuitry is triggered from the falling edges of the initial encoder-derived wavetrain. The alternating opaque markings and transparent segments (or solid bars and orifices) of the encoder strip are arranged in time alignment with the signals that result from reading of those features by a transmissive optical emitter/detector pair.
It will be understood that, in selecting the point at which a mark should be made, it is possible to make allowance for the nominal width of the transparent segment. For example, the firing of a pen could be delayed by a period of time automatically calculated from the nominal width of the transparent segment divided by the carriage velocity. Although both these pieces of information are available during operation of the system, the results of this method would be unsatisfactory because of preferred manufacturing procedures for creation of the encoder strip . These procedures arise from economics related to dimensional requirements, as follows.
Thus, in making a encoder strip, the dimension which is most important to hold to highest precision is the overall periodicity of the alternating opaque bars and transparent segments--i.e., the periodicity dimension that gives rise to a full wavelength of the wavetrain. The two internal dimensions of each mark-and-transparent-segment pair--namely, the length of the bar and the length of the transparent segment--are much less important.
In a unidirectional printing machine, only the distance between falling edges (or alternately rising edges) has any importance, provided only that (1) the distance from each falling edge to its next associated rising edge is great enough to permit the sensing apparatus to recognize the falling edge; and (2) the distance from each rising edge to its next associated falling edge is great enough to permit the sensing apparatus to reset itself in preparation for sensing the failing edge.
More specifically, the dimensional accuracy of the encoder-strip features are plus-or-minus only one percent for the full periodic pattern width, but plus-or-minus ten to twenty percent for the opaque bar width alone. While it would be entirely possible to manufacture an encoder strip with much finer precision in the internal dimensions just mentioned, an encoder strip so made would be substantially more expensive.
(b) Time-of-flight and analogous misalignment effects--A certain amount of time elapses between the issuance of a mark-command pulse to a print head and the mark actually being created on the printing medium. For instance, in an inkjet printer, some time elapses between: the issuance of a fire-command pulse--approximately at an encoder-wavetrain falling edge--to a pen nozzle and the instant when a resulting ink drop actually reaches the medium.
During this time, however, the carriage and pen continue to move across the printing medium--and, in the case of an inkjet device, so does the ink drop, even after leaving the pen. The initial velocity component of the drop along the scanning axis or dimension, when scanning forward, is very closely equal to the carriage velocity; this velocity likely decreases while the drop travels in the orthogonal axis or dimension toward the printing medium--but nevertheless, some forward movement or displacement of the ink drop along the scanning axis does occur before the drop reaches the medium to form an ink spot.
In a unidirectional printing machine, this delay is substantially inconsequential, once the carriage velocity of the pen is constant, for all the ink drops are offset in this same manner by very nearly the same distance, and in the same direction. In other words, the entire image is offset together along the scanning axis; but this does not matter to the resulting printed image because there are no relative offsets within the image--and therefore no discontinuities, no distortions of image features, etc.
Thus time-of-flight and analogous misalignment effects impede the creating high-accuracy images. While these effects are substantially independent of the imprecisions discussed in the preceding section, it should be recognized by those skilled in the art that in order to reduce printer cost and improve throughput, these two factors are critical.
Therefore it would be highly desirable to have a new and improved system that corrects for variations in the ink drop flight path due to changing carriage velocity, and that also adjusts the extrapolated positions at which drops are fired to account for a non-constant carriage velocity.
In order to have precise positioning information, other factors, such as carriage velocity and acceleration must also be considered. The relationship between position and movement factors will now be expanded to provide a better understanding of the advantages of the present invention.
As discussed earlier a well-known way to provide the position and speed information is by means of at least one electro optical sensor that is moved in accordance with the print-head or reading-head movement, and that monitors a so-called "encoder scale". Because plural-sensor systems are more complex and expensive than single-sensor systems, the present invention is limited to single-sensor systems.
In either a single-or a plural-sensor system, the scale is disposed in correspondence with positions (that is, the full range of positions) of the head across the image-bearing sheet. Such a scale generally takes one of two forms: (1) a linear strip (often denominated "codestrip") extended--and usually tensioned--across a bed or channel that holds the image-bearing sheet, the strip being directly adjacent and parallel to the print-head or reading-head motion; and (2) a circular, hub-mounted scale read against--for example--the shaft of a motor that drives the print head.
Every such system has some arrangement for initializing the counting of graduations, starting precisely at a well-defined edge of the image area. Counting then continues across that area within a controlled range of speed so that the automatic equipment can, in effect, lock onto the progressively changing position. Once initialized, the system can maintain this lock as long as movement continues in the same direction.
It will be understood that for fine positional precision the graduations are spaced very closely, and accordingly each graduation must necessarily be very narrow. The stopping distance and precision of the marking or reading head--for rapid scanning such as called for by high throughput--is not readily made equal to a small fraction of the periodicity of these fine graduations.
Therefore the ambiguity cannot be easily resolved by design adjustment of the relative magnitudes of graduations vs. stopping precision.
One well-known way to resolve the ambiguity is to provide not just one but two sensors, both reading the same codestrip but mutually offset along the line of motion by a known distance. In particular it is known in such a so-called "dual-channel encoder" to offset the two sensors by one-quarter of the overall periodicity of the graduations on the encoder scale (or by that distance plus or minus an integral number of periodicities), resulting in two electrical pulse trains in quadrature.
The velocity and acceleration of the print-head is then ascertainable automatically through comparison of the two pulse trains. Such systems work well but are objectionably expensive in that they require an additional sensor and associated electronics.
Another method has been to provide a circuit to estimate the velocity of the pen carriage and then shoot the drops of ink at the time when the carriage is predicted to be in the proper position. This process has historically been called position extrapolation: predicting future positions based on the time of occurrence of past positions. Based on the current estimate of the pen carriage velocity, a circuit is able to create a sequence of "subpulses" that occur when the carriage is expected to be in positions that are very close together. These subpulses provide a positional reference estimate that is much higher in resolution than the original encoder pulses.
Another practice of past inkjet printers has been to print only when the pen carriage is sweeping at a constant velocity. In such system, the carriage must accelerate from being stationary to traveling at the set print speed before printing may begin for a given sweep. This acceleration requires both time (which slows down printer performance by limiting the number of pages that can be printed in a fixed amount of time) and travel distance (which forces the printer to have added width making the product much larger).
To reduce printer cost and improve throughput, it would be highly desirable to have a new and improve printer and method of printing that permits printing while the carriage is accelerating. Such a new an improved printer should also accurately determine ink droplet positioning to ensure that such droplets hit the medium at a proper position.